documentclass{article}
usepackage{expl3}
begin{document}
  ExplSyntaxOn
  tl_new:N __a
  tl_new:N __b
  tl_set:Nn __a { emph{emph} }
  tl_set:No __b { exp_after:wN overset
                    exp_after:wN {
                    exp_after:wN o
                    exp_after:wN }
                    exp_after:wN { __a }  }
  tl_show:N __b % target: b = overset{o}{emph{emph}}
end{document}

tl_set:Nn l_tmpa_tl { emph { emph } }
tl_set:Nx l_tmpb_tl
  { overset { o } { exp_not:V l_tmpa_tl } }